E. R. Saundbes 337 



viz. 1 in 4. This appears undoubtedly to be the case in the great 

 majority of families, but there are a certain number of cases in 

 which a considerably lower percentage of doubles was obtained, while 

 occasionally the proportion was in excess of this amount. The cases in 

 which the proportion of doubles is very small are so marked and, in 

 some unions, of such frequent occurrence, that it seems clear that they 

 cannot be regarded as other than genuine — that they must in fact 

 represent a distinct ratio and not an extreme variation from the usual 

 3 s. : 1 d. On the other hand it seems highly probable that in the one 

 or two cases where the proportion of doubles recorded is distinctly 

 higher than 1 in 4 the excess is accidental. 



It is the frequent occurrence of numbers approximating to the ratio 

 3 s. : 1 d. which suggests that, in the wo-d-strains, the factors for 

 singleness {X and Y) are so coupled that re-combination with x and y 

 in the gametogenesis of ^i cannot occur, in the manner described 

 above for matings between two eversporting forms, where ovules with 

 X and Y uncoupled meet xy pollen grains. This condition of single- 

 ness which is typical of non-double-throwing forms is, as stated above, 

 conveniently represented thus XY: and since as regards singleness 

 and doubleness, reciprocal heterozygotes of similar composition give 

 similar results, we may write XY for both the ovules and the pollen 

 of a typical no-d-ioTxn. But, as explained above (p. 335), a single 

 might breed true and yet not be pure-bred, a fact which should 

 not be overlooked in considering any unexpected result in F^. For in 

 any cross between an eversporting and a true-breeding type, made 

 in the form rf $ x no-d (^, a certain number of the F^ singles will have 

 the composition XY XY; they will breed true to singleness, and on 

 self-fertilisation will be indistinguishable from a pure-bred true-breeding 

 single having the composition XY XY. But the cross-bred true-breed- 

 ing single will presumably behave differently from a typical pure-bred 

 single, when crossed with an eversporting form; since in the one mating 

 the xy germ cells of the d-type will unite with certain germ cells in 

 the no-€?-type carrying an uncoupled XY group, and in the other, not. 

 We have already seen that of the double-throwing plants assumed at 

 first to be pure-bred eversporting, some were probably cross-bred ^ and 

 it may well be that now and again the same may be found to be the 

 case with a supposed type single. From such a true-breeding but 



1 See pp. 313 — 317 where an account is given of the behavioor of commercial samples 

 of the white and cream strains. 



